call them concerning payments made

Over the last few months I have had to call them concerning payments made to them that they have not posted to my account correctly or not at all and yet have cashed from my bank. when I contact customer service they say they can’t actually look the checks up and that it must be my fault for not writing the correct info on my checks. I do exactly as they tell me and it just keeps happening. But the most frustrating part is that they mark my account past due! I am never past due on my bills and have a high credit rating because of that. I don’t think it is fair for a company to get away with what they are doing. If they have my money then my account should not reflect past due!